transicion vitrea cafe

8/2/2019 Transicion Vitrea Cafe

1/5

Eur Food Res Technol (2005) 221:434438DOI 10.1007/s00217-005-1196-2

O R I G I N A L P A P E R

Monica Anese Lara Manzocco Enrico Maltini

Effect of coffee physical structure on volatile release

Received: 19 November 2004 / Revised: 14 March 2005 / Published online: 21 July 2005 Springer-Verlag 2005

Abstract The mechanisms of volatile release from sol-uble coffee powders with different roasting degrees werestudied. The presence of volatiles in the headspace during

coffee humidification was analyzed by gas chromatogra-phy. Small amounts of volatiles were observed at lowwater activities (aw), independently from the roastingdegree; as the aw increased headspace volatiles rose andthen decreased as the moisture further increased. Thechanges in aw and volatiles went along with the changesof coffee structure from a free-flowing powder to a stickyviscous fluid. The mechanism of volatiles release wascontrolled by both kinetic and thermodynamic factors.The former prevailed at low aw in glassy systems; whilethe latter became important at high aw when, due to in-creased mobility, equilibrium conditions were ap-proached. Modified state diagrams were used to predict

the critical temperature and aw at which structural col-lapse and volatile release occurred. As far as equilibriumcondition was achieved, coffee volatiles were partially re-adsorbed in the liquid phase.

Keywords Coffee powder Volatile release Physicalstructure Phase transitions

Introduction

Flavor is one of the most important attributes of foodquality and acceptability. The partition of volatile com-ponents between the gas phase and food is relevant to theresulting quality [1]. However, as partitioning phenome-non is mainly controlled by interactions between volatileand nonvolatile components, control or prediction offlavor release can be difficult [2]. For instance, manu-

facturing and stabilization processes, as well as domesticmanipulation, may often cause modification in the flavorpartition thus affecting aroma intensity or profile [1, 3, 4].

Although not all volatiles are flavor components, an un-derstanding of the mechanisms involved in volatile re-tention and release by a food matrix may be required for abetter control of sensory quality of foodstuffs [5].

Many literature studies deal with the volatile retentionin low moisture food matrices [610]. The high degree ofvolatile retention in dehydrated foods, particularly infreeze-dried products, contrasts with the behavior ex-pected for organic compounds, considering their rela-tively high volatility compared to that of water [5, 7, 1018]. This deviation from the expected behavior has beentaken, since the earliest investigations, as a physically andstructurally based phenomenon [1922]. According to

these theories, volatile compounds are entrapped into thedehydrated matrix and are released during moistening dueto structural collapse of the matrix itself.

In the end of the 1980s, by introducing the concept ofglass transition, it has been assumed that dehydratedfoods can be considered as amorphous, metastable solids.When the residual moisture is low enough, amorphousmaterials may exist in a glassy state at room temperature.In these conditions physical changes and related phe-nomena depend on kinetic rather than thermodynamicmechanisms, and their occurrence can be predicted withreference to the glass transition temperature (Tg) [23, 24].According to this interpretation, volatiles are entrapped inthe amorphous glass, where diffusion is very low. Plas-ticization by absorption of water may cause the depres-sion ofTg below room temperature, and hence the glass-rubber transition of the matrix. In these conditions thestructural change of the matrix may allow initial collapseto occur and volatiles to be released [2528].

Following the hypothesis mentioned above, the ob-jective of the present work was to investigate on the roleof structure transitions in the volatile release from in-stant coffee, a well-known example of food matrixcharacterized by high volatile retention.

M. Anese ()) L. Manzocco E. MaltiniDipartimento di Scienze degli Alimenti,University of Udine,Via Marangoni 97, 33100 Udine, Italye-mail: [email protected].: +39-0432-590711Fax: +39-0432-590719


2/5

Materials and methods

Coffee powders

Light-, medium-, and dark-roasted soluble coffee powders wereobtained by extraction and spray-drying from a same blend. Ac-cording to the supplier (Nestl Research Centre of Lausanne), thepowders were characterized by color test Neuhaus (CTN) values of110, 85, and 60, and weight losses of 14.2, 16.2, and 18.9%, re-spectively for the light, medium and dark types.

Sample preparation

Aliquots of 25 g of each soluble coffee were solubilized with 25 gof deionized water, freeze-dried (Mini Fast mod. 1700, EdwardsAlto Vuoto Spa, Trezzano sul Naviglio, Milano, Italy) and storedunder P2O5 for few days before further manipulations. The sameprocedure was followed for a light-roasted coffee sample addedwith 2% (w/w) ethanol (Carlo Erba, Milano, Italy) before freeze-drying.

a. Weighed vials (10 mL capacity) containing approximately 1 gof freeze-dried powders were equilibrated in vacuum dessica-tors containing saturated salt solutions (Carlo Erba, Milano,Italy), having equilibrium relative humidities (RH) ranging

from 11 to 94%, and maintained at 25 C. After 1 week,equilibrium conditions were reached and vials were hermeti-cally closed with butyl septa and metallic caps.

b. About 2 g of the freeze-dried light-roasted coffee containingethanol (2% w/w) were introduced in 300 ml-capacity glassflasks over saturated CuCl2 (68% RH) solutions. Analysis wasperformed over the equilibrium time of 120 h.

Headspace gas-chromatographic analysis

Analyses were performed on samples conditioned at 25 C in atemperature-controlled water bath by using a Fisons gas-chro-matograph (HRGC MEGA 2 series, Fisons Instruments, Milano,Italy), equipped with an automatic sampler and a flame ionizationdetector (Carlo Erba HS 250, Carlo Erba Strumentazioni, Milano,

Italy). A 2 mm2 mm i.d. glass-packed column filled with Car-bopack 20 M 80/100 mesh (Waters Association, Framingham,Massachusetts, USA) was used. The operating conditions were asfollows: column temperature, 80 C; detector oven temperature,200 C; injector temperature, 200 C; carrier gas (nitrogen) flowrate, 35 mL/min. The headspace volume injected was 0.5 mL usinga precision sampling syringe (Dynatech Precision Sampling, BatonRouge, Louisiana, USA), provided with a pressure lock and a gasvolume capacity of 00.5 mL. The chromatograms were analyzedusing a ChromCard for Windows software (Carlo Erba Instruments,Milano, Italy).

Ethanol vapor pressure (Pe) was calculated from the chro-matographic response of the ethanol peak area by using the Kolbequation [29]:

P PoA

Ao

where Po

is the vapor pressure of pure ethanol, equal to 7.45 kPa at25 C; A is the ethanol peak area; A

ois the pure ethanol peak area.

Calorimetric analysis

Calorimetric measurements were made by means of a TA 4000differential scanning calorimeter (Mettler Toledo, Greinfensee,Switzerland), equipped with a DSC 30 low-temperature measuringcell and connected to a Graph Ware software (TA 72.2/.5,Switzerland). Heat flow was calibrated using indium (heat of fusion28.45 J/g). Temperature was calibrated with n-butyl alcohol (m.p.

89.5 C), water (m.p. 0 C) and indium (m.p. 156.6 C). Amountsof about 10 mg of coffee samples, were placed into 40 mL alumi-num DCS pans closed with pressure sealing. An empty aluminumpan was used as a reference. The glass transition (onset Tg) tem-peratures of the anhydrous powders were measured by heating from100 to 200 C at 10 C/min. The transition temperatures of thecoffeewater mixtures were determined after rapid cooling of thesamples to 100 C, heating at a scanning rate of 10 C/min up tothe onset of the melting endotherm, re-cooling at 100 C andscanning to 200 C at 10 C/min.

Total solid content

Total solid content was determined according to AOAC metho-dology [30].

Sorption isotherms

Sorption isotherms were measured by accurately weighed samplesin vacuum dessicators containing saturated salt solutions withconstant water vapor pressure until constant weight [31]. Mea-surements were carried out at 25 C. Saturated salt solutions withRH between 11 and 94% were used.

Water activity

Water activity (aw) was determined by means of a dew-pointmeasuring instrument (AQUA LAB, Decagon, Pullman, WA,USA) at 25 C.

Microscopic analysis

Coffee samples were analyzed by means of optical microscopy(Wild Makroskop, M420, Switzerland) with image magnificationequal to 20. Images were captured with a video camera (ColorVideo Camera Head, JVC, Model TK 1280Z, Japan), and elabo-rated using the software Adobe Photoshop (6.0, Adobe SystemsInc., 2000).

Data analysis

The results reported in this work are the average of at least threedeterminations and the coefficients of variation, calculated as thepercentage ratio between the standard deviation and the meanvalue, were less than 7% for calorimetric analyses, less than 5% forgas-chromatographic and total solid content determinations and lessthan 3% for water activity.

The significance of the differences among means was deter-mined using the TukeyKrammer test [32]. Means were consideredto be significantly different at P


3/5

Results and discussion

Figure 1 shows the equilibrium headspace volatile area,measured at 25 C, of light-, medium- and dark-roastedcoffees as a function ofaw. As expected, the presence ofvolatiles in the headspace of coffees increased with theincreasing of the roasting degree [35]. At both low and

high aw, the headspace area of volatile compounds wasvery low, whereas the peak areas were highest at inter-mediate aw. The aw associated with the increase of vo-latiles in the headspace was in the range of 0.250.35 forthe light- and medium-roasted coffees, while a high vol-atile area for the dark-roasted one was observed up to anaw of about 0.7. Thus, the presence of volatiles in theheadspace showed a bell-shaped behavior for all the

coffee samples. High volatile retention at low aw valueswas also observed by Flink and Karel [7] on aqueousmodel systems containing soluble carbohydrates and or-ganic volatiles. According to these authors, in such sys-tems volatile compounds would be entrapped in mi-croregions formed by carbohydratecarbohydrate hydro-gen bonds. A bell-shaped behavior was also reported byGunning et al. [9], although their study was more focusedon the effect of water content on volatile release.

In coffee samples, the changes in headspace partitionof volatiles were associated with visible changes in thephysical structure of the powders (Fig. 2). By examiningthe images of the light-roasted coffee samples equili-

brated at different aw (Fig. 2), it is possible to have anidea of their physical structure. At very low aw thestructure was that of a free-flowing powder. At aw of 0.33,where the headspace area of volatile compounds was

Fig. 1 Total peak area of headspace volatile compounds of light-('), medium- (n) and dark-roasted (l) soluble coffees as a func-tion ofaw

Fig. 2 Photographs (20) of light-roasted coffee samples having different aw (A, aw=0.05; B, aw=0.33; C, aw=0.48; D, aw=0.70)

436


4/5

high, the powder structure could be described as a hy-drated amorphous mass, probably because of the initialformation of liquid bridges between the coffee particles[36]. As the aw value further increased (aw>0.44), with acorresponding decrease of peak area, the coffee matrixturned into a sticky and fully collapsed structure. As

known, collapse or structural sinking is a time dependentphysical change, which occurs when viscosity is reducedto a value at which the porous matrix cannot sustain itsown weight against gravity [37, 38]. As mentioned above,volatile compounds may be released upon collapse.However, in the latest stage of collapse a drastic decreasein porosity occurs and diffusion of the residual volatilesmay become strongly hindered. In any given system, thetemperature at which collapse may occur depends onwater content and solute nature: it decreases with theincrease of water content and the decrease of solute av-erage molecular weight [37]. In structurally simple sys-tems (e.g., sugar solutions) the collapse temperature co-

incides with Tg; in more complex systems, containingsuspended matter, or viscosant components (hydrocol-loids, starch, etc.), or when an insoluble cellular matrix ispresent, the apparent collapse temperature may be higherthan Tg.

Figure 3 shows a modified state diagram relative to thelight-roasted coffee. In principle, modified state diagrams,where Tg and water content are plotted against aw, allowthe aw/temperature conditions at which the system mightundergo glass transition to be predicted [39]. Accordingto Fig. 3, at room temperature (e.g., 25 C) the solublecoffee powder is in a glassy state at aw lower than about0.35; over that critical value, the transition to a viscousrubbery state may allow matrix collapse to initiate andvolatiles to be released. Such transition may thus be as-sociated to an increase of both temperature and/or watercontent. By contrast, storage of coffee at lower tempera-tures would allow collapse and thus volatile release tooccur at higher water content and aw. Similar diagrams(not shown) were obtained for the medium- and dark-roasted coffees. In the experimental conditions, coffeesTg (Table 1) as well as water sorption data (data notshown for medium- and dark-roasted coffees) were notsignificantly affected by the roasting degree (P>0.05),although the amount of volatiles was much higher in the

dark-roasted coffee. Taking into account that coffeepowder was only slightly hydrated at the moment ofmaximum amount of volatiles in the headspace (Fig. 2)and that the glass transition spanned over a rather largetemperature range (Table 1), it is likely that most of thevolatile compounds are released in the first stage of col-lapse, just above the onset of the glass transition.

The decrease of volatiles after the maximum requiresan explanation. In principle it could be hypothesized thatat high aw the kinetics of volatile release is slower thanthat of collapse, and volatiles can no more escape from afully collapsed matrix. Alternatively, a volatile re-ad-sorption by the salt solution and/or the product itselfshould be envisaged.

To check the above hypothesis, the kinetics of volatilerelease during equilibration to a constant 68% RH wasevaluated on a light-roasted coffee sample added with 2%(w/w) ethanol, which was shown to not affect the Tg ofthe coffee sample. Samples were analyzed for volatilesand aw at time intervals during equilibration. Figure 4shows the nonequilibrium headspace total peak area as afunction of time. Again, a bell-shaped curve, with amaximum at about 0.44 followed by a decrease was ob-served, which also in this case was associated to a stickyand collapsed structure. This behavior confirms that suf-ficiently hydrated sample and/or salt saturated solutionsare able to partially re-adsorb volatiles, when the kineticeffect of the matrix structure becomes less important dueto the increased mobility.

To evaluate the matrix effect, the behavior of coffeesamples added with 2% (w/w) ethanol and equilibrated atdifferent aw was compared with the results of a similar

Table 1 Onset and endset of the glass transition temperature (Tg),glass transition temperature of the maximally freeze-concentratedsolute T0g

for light-, medium- and dark-roasted coffee samples

Roastingdegree

Tg (onset)(C)

Tg (endset)(C)

T0g

(C)

Light 66 83 39Medium 67 78 39Dark 65 80 39

Fig. 3 Modified state diagram of light-roasted coffee

Fig. 4 Total peak area of headspace volatile compounds of light-roasted soluble coffee added with 2% (w/w) ethanol as a function ofequilibration time to 68% RH. aw values at corresponding equili-bration times are also reported

437


5/5

experiment, taken from Lerici et al. [29], on a waterglycerolethanol system. Figure 5 shows that, in contrastto the bell-shaped behavior of the coffee-based system,the ethanol vapor pressure over the waterglycerol systemlinearly decreased as the aw increased. Actually, the

glycerolwater system is a low viscosity liquid wherethermodynamic equilibrium spans over the entire awrange and liquidvapor partition obeys to the Raoultslaw.

In light of these results, it can be concluded that, at awvalues lower than the critical one, the coffee volatile re-lease in the headspace is under kinetic control. On thecontrary, when coffee samples have passed through andovercome a critical aw, the kinetic constraints are pro-gressively reduced and the system tends to the attainmentof a thermodynamic equilibrium. The latter implies vol-atile partition between liquid and vapor phases accordingto the Raoults law.

Acknowledgements Nestl Research Centre of Lausanne isthanked for providing the coffee samples

References

1. Lubbers S, Landy P, Voilley A (1998) Food Technol 52(5):6874, 208214

2. Druaux C, Voilley A (1997) Trends Food Sci Technol 8:364368

3. Hofmann T, Czeny M, Calligaris S, Schieberle P (2001) J AgricFood Chem 49:23822386

4. Lopes da Silva JA, Castro SM, Delgadillo I. (2002) J AgricFood Chem 50:19761984

5. Goubet I, Le Quere J-L, Voilley AJ (1998) J Agric Food Chem46:19811990

6. Flink J, Karel M (1970) J Agric Food Chem 18:2952977. Flink J, Karel M (1972) J Food Technol 7:1992118. Flink J, Gejl-Hansen F (1972) J Agric Food Chem 20:691694

9. Gunning YM, Gunning PA, Kemsley EK, Parker R, Ring SG,Wilson RH, Blake A (1999) J Agric Food Chem 47:51985205

10. Rifa MK, Voilley A (1991) J Food Sci Technol 28:13513711. Lerici CR (1976) Rivista Scientifica di Tecnologia, Alimen-

tazione e Nutrizione Umana 6:273112. Aguilera JM (2000) Structure-properties relationships in foods.

In: Lozano L, Anon C, Parada E, Barbosa G (eds) Trends inFood Engineering. Technomics, Lancaster, PA, pp 114

13. Blake A (1994) Int Food Ingredient 3:303414. Omatete OO, King CJ (1978) J Food Technol 13:265280

15. Rosenberg M, Kopelman IJ, Talmon Y (1990) J Agri FoodChem 38:12881294

16. Thijssen HAC (1971) J Appl Chem Biotechnol 21:37237717. Voilley A (1995) Flavor encapsulation. Influence of encapsu-

lation media on aroma retention during drying. In: Risch SJ,Reineccius GA (eds) Encapsulation and Controlled Release ofFood Ingredients. American Chemical Society, Washington,DC, pp 169179

18. Whorton C (1995) Factors affecting volatile release from en-capsulation matrices. In: Risch SJ, Reineccius GA (eds) En-capsulation and Controlled Release of Food Ingredients.American Chemical Society, Washington, DC, pp 134142

19. Chirife J, Karel M, Flink J (1973) J Food Sci 38:67167420. Flink J (1975) The retention of volatile components during

freeze-drying: a structurally based mechanism. In: GoldblithSA, Rey L, Rothmayr WW (eds) Freeze Drying and Advanced

Food Technology. Academic Press Inc., London, pp 798821. To EC, Flink JM (1978) J Food Technol 13:56758122. To EC, Flink JM (1978) J Food Technol 13:58359423. Levine H, Slade L (1986) Carb Polym 6:21324424. Levine H, Slade L (1988) Cryo-Lett 9:216325. Labrousse S, Roos Y, Karel M (1992) Science des Aliments

12:75776926. Levi G, Karel M (1995) J Food Eng 24:11327. Shimada Y, Roos Y, Karel M (1991) J Agric Food Chem

39:63764128. Whorton C, Reineccius GA (1995) Evaluation of the mecha-

nisms associated with the release of encapsulatated flavor frommaltodextrin matrices. In: Risch SJ, Reineccius GA (eds) En-capsulation and Controlled Release of Food Ingredients.American Chemical Society, Washington, DC, pp 143160

29. Lerici CR, Nicoli MC, Manzocco L (1996) Industrie Alimentari

35:131330. AOAC (1995) Official Methods of Analysis 16th ed. Associa-tions of Official Analytical Chemists International, Arlington,VA. Chap.42

31. Gal S (1975) Recent advances in techniques for the determi-nation of sorption isotherms. In: Duckworth RB (ed) WaterRelations in Foods. Academic Press, London, pp 139154

32. SAS (1999) JMP 3.2.5. SAS Institute Inc., Cary, NC33. Gordon M, Taylor JS (1952) J Appl Chem 2:49350034. Johari GP, Hallbrucker A, Mayer E (1987) Nature 330:55255335. Grosh W (2001) Chemistry III: volatile compounds. In: Clarke

RJ, Vitzthum OG (eds) Coffee Recent Developments. Black-well Science Ltd., Oxford, UK, pp 6889

36. Peleg M (1983) Physical characteristic of food powders. In:Peleg M, Bagley EB (eds) Physical Properties of Foods. AVIPublishing Company Inc., Westport, CT, pp 293323

37. Tsourouflis S, Flink JM, Karel M (1976) J Sci Food Agric27:509519

38. Levi G, Karel M (1995) Food Res Int 28:14515139. Roos YH (1995) Phase Transitions in Foods. Academic Press,

New York

Fig. 5 Ethanol vapor pressure as a function of aw of ethanol-con-taining light-roasted coffee (') and water-glycerol systems (n)

438

transicion vitrea cafe

Documents